1. Field of the Invention
The invention relates generally to ultrasonic and radiographic non-invasive methods for examining tissue or other solids. In particular, the invention relates to the coordination or fusion of ultrasonic sonograms with x-ray or other radiographic imaging techniques, to aid in the detection of areas of interest, such as lesions in a human breast.
2. Description of the Related Art
Various means of non-invasive imaging are useful in medicine and other fields for visually modeling the interior structure of a solid subject body. For example, a very common method of screening women for breast cancer is x-ray mammography. Ultrasonic imaging is another, less common technique for examining breast tissue.
X-ray mammography provides excellent detection of certain types of tissues, but nevertheless has shortcomings. This technique provides detailed image information about dense materials within the body (such as bone or other calcified tissue), but it performs poorly at discriminating between soft tissues with subtle differences in density and structure. Some women have mammographically dense breasts (compared to more fatty breast tissue); there is a substantially increased risk of missing breast cancers when diagnosing such women by x-ray. The use of x-rays for examination also necessarily results in the exposure of the patient to ionizing radiation, which has well know associated risks. The technique is also limited in that it projects three-dimensional structure onto a two-dimensional plane, and thus does not capture the elevation or depth (position in the direction of radiation propagation) of features of interest.
A newer imaging technique, ultrasonic imaging, is widely used for diagnosis in numerous medical fields. When properly used and adjusted, an ultrasound imaging system can non-invasively provide a cross-sectional view of soft tissue being imaged, such as the tissue of a breast, heart, kidney, liver, lung, eye, abdomen, or pregnant uterus.
A typical ultrasound imaging device operates by directing short ultrasonic pulses, typically having a frequency in the range of 1-30 MHZ, into the tissue being examined. The device then detects responses such as echoes, harmonics, phase or frequency shifts, of the ultrasonic pulses caused by acoustic impedance discontinuities or reflecting surfaces within the tissue.
A typical scanhead for an ultrasonic imaging system has a linear array of ultrasonic transducers which transmit ultrasonic pulses and detect returned responses. The array of transducers provides simultaneous views of the tissue at positions roughly corresponding to the positions of the transducers. The delay time between transmitting a pulse and receiving a response is indicative of the depth of the discontinuity or surface which caused the response. The magnitude of the response is plotted against the position and depth (or time) information to produce a cross-sectional view of the tissue in a plane perpendicular to the face of the scanhead.
Sophisticated ultrasonic imaging systems are available which are capable of assembling information from multiple two-dimensional cross-sections to form a three dimensional representation of subject tissue. Such systems are potentially useful in the diagnosis of suspicious lesions in the breast. Compared to x-ray techniques, the ultrasonic imaging is advantageous in that the patient is not exposed to radiation. Ultrasound is also superior for imaging many types of soft, low-density xe2x80x9chidden massesxe2x80x9d which are typically invisible or very obscure in x-ray imagery. On the other hand, the lower resolution of ultrasonic imaging (compared to x-ray) makes it incapable of identifying fine features, such as hard micro calcifications in breast tissue, which would be visible in an x-ray.
Although a patient (or other subject body) can be subjected to multiple imaging techniques (for example x-ray and ultrasound), the images obtained are not easily registered or correlated with one another. Differences in scale, position, or in the orientation of the plane of projection (of a two-dimensional image) are almost inevitable.
U.S. Pat. No. 5,531,227 to Schneider (1996) discloses a method and apparatus for obtaining an image of an object obtained by one modality such that the image corresponds to a line of view established by another modality. However, the method disclosed requires one or more fiducial markers to inter-reference the images. The preferred method disclosed also involves mounting the patient""s head immovably to a holder such as a stereo tactic frame, which is inconvenient for the patient and the technicians. The method identifies fiducial markers by digital segmentation, feature extraction, and classification steps, which would most suitably be performed with powerful digital hardware and custom software. The method disclosed will perform best with fiducial markers which are easily automatically recognized, as by some simple geometric property; it is described in connection with using circular eye orbits as fiducial markers. In some human tissues, however, such natural geometric features may not be readily available.
Another method of correlating ultrasonic image data with radiographic image data is disclosed in U.S. Pat. No. 5,640,956 to Getzinger et al (1997). This method requires that an x-ray image be obtained while the tissue is in the same position as it was while the ultrasonic data was being gathered. It also requires the use of fiducial reference markers (preferably multiple x-ray opaque reference markers).
The invention is an apparatus and method for quickly coordinating ultrasonographic information about the internal structure of a solid subject body with x-ray or other radiographic information taken from the same subject body.
Given a radiographic transmission image of a subject body, and given further a set of three dimensional image data composed of tomographic slices of the same subject body, the invention relates a region in the original radiographic image to a region within the three-dimensional image data by using two-dimensional image cross-correlation, preferably performed by an optical correlator. In the preferred embodiment, the invention also uses a two-dimensional cross-correlation to find the elevation of a feature of interest in the three-dimensional data set.
Provided that the differently obtained images include at least some overlap, it is not required that the ultrasonographic and the radiographic images share identical or previously related scales, positional axes or spatial orientation. They may be taken from different angles, at different scales, or with the subject positioned and rotated differently in each image. The invention determines and applies the translations, rotations and scale transformations required to best correlate the radiographic information with the ultrasonographic information corresponding to the same physical region and view of the subject body.